Interpretation of helium and neon isotopic heterogeneity in Icelandic basalts

Abstract

New noble gas data from stepwise heating of nine samples from the Reykjanes Peninsula, Iceland, show total gas 3He/4He ratios ranging from relatively low values of 11±1 R/Ra to higher values of 21±2 R/Ra (where Ra=atmospheric 3He/4He ratio). These values are similar to previously measured ratios in Icelandic basalts, and higher than the average mid-ocean ridge basalt (MORB) 3He/4He ratio of ∼8.5 R/Ra. Significantly, new neon isotopic data from five of these samples lie on, or above, the MORB trend in the neon three-isotope plot. These data provide the first strong evidence for a MORB-like neon isotopic component in Icelandic basalts. The remaining data lie on or near the air–solar mixing line, or within 1σ of the atmospheric ratio. The combined new and previously published Icelandic mantle neon isotopic ratios range from MORB-like to solar-like compositions. The possible origins of this significant noble gas isotopic heterogeneity are evaluated in terms of two previously published models, the plume source model and the binary mixing model [Moreira et al., Earth Panet. Sci. Lett. 185 (2001) 15–23]. The plume source model is evaluated by comparison of the calculated 3He/22Ne elemental ratios from Iceland with the published estimates of the 3He/22Ne ratios in the solar nebula (1.9) and MORB (∼10). Most Icelandic 3He/22Ne (calculated) ratios are between ∼1 and 12 and suggest the Icelandic plume source is quite heterogeneous, but alternative explanations are possible. The heterogeneous Icelandic helium and neon isotopic ratios that give rise to the range in calculated 3He/22Ne ratios are also consistent with binary mixing between solar-like Icelandic plume- and MORB-derived noble gases. These end-members have specifically defined isotopic ratios but elemental ratios that may be variably modified. A large degree of recent relative elemental fractionation of He/Ne ratios in the Icelandic plume and MORB end-members would be needed to produce the observed range in isotopic ratios. The binary mixing model was further evaluated by comparison of Icelandic data with those from the Shona Ridge, a locality where binary mixing is believed to occur. In a plot of calculated 3He/22Ne and 21Ne/22Ne ratios, the Icelandic and Shona Ridge data form divergent linear trends that can be reconciled with binary mixing and that, interestingly, intersect at values of 1.6 and 0.036, respectively. These values are similar to the estimated solar nebula composition (1.9 and 0.0328). Such low calculated 3He/22Ne ratios have not been determined previously in terrestrial mantle-derived samples and may suggest that the plume source contains a component that is relatively unfractionated compared with the primordial solar nebula composition. Published steady-state upper mantle models require that the 3He/22Ne ratios in the plume (lower mantle) and MORB (upper mantle) sources are about equal to each other. The calculated 3He/22Ne ratio of near 1.6, inferred to represent the Icelandic and Shona Ridge mantle plume source composition, is dissimilar from the estimated MORB 3He/22Ne ratio of near 10. This appears to suggest the upper mantle is not at steady state with respect to the plume source mantle. The results of this study do not rule out the possible existence of Icelandic plume source noble gas elemental and isotopic heterogeneity. However, they do suggest that evidence for plume source heterogeneity may be obscured as a result of relatively shallow-level elemental fractionation and binary mixing processes.